In a basic cellular telecommunication system, a mobile switching center (MSC) is linked to a network of base stations by a series of digital transmission links. The base stations are geographically distributed to form an area of coverage for the system. Each base station is designated to cover a specified area, known as a cell, in which a two way communication can take place between a mobile station and the base station in the associated cell. The mobile station operating within a cell communicates with the base station over the air interface on a specified radio channel. For simplicity, the term mobile station will henceforth be referred to simply as the mobile.
Manufacturers of wireless telecommunication equipment have long recognized the importance of testing the equipment prior to installation in the field. Many of the tests performed. on the base station equipment and the mobile switching center (MSC) are typically well developed and do a good job of simulating real-life conditions. Other areas, such as those involving the air interface, have generally not been tested as rigorously, partly due to the inherent environmental randomness associated with the radio path thus leading to a lack of repeatability in testing procedures. For example, the actual environment a service provider operates in can contain a number of factors that are difficult to recreate in a test environment such as changing weather conditions, interfering signals, log normal and multi-path fading from obstacles and/or terrain etc. The precise conditions are often difficult to recreate in a laboratory back at the manufacturing site.
A further complicating factor is that there is a limit to the amount and types of tests that can be performed at the installation site without significantly affecting current service. Since the environmental conditions at an operating site cannot be guaranteed to remain consistent, it may be difficult to compare results from repeated tests with much confidence. Thus the system reliability and robustness of the controlling software become difficult to verify from repeated testing where conditions need to be held constant.
In prior testing methods, there have been attempts made to simulate the air interface in testing procedures in a laboratory environment. One example known in the art is to use a coaxial cable to connect the mobile to the base station to form a coaxial network air interface. This type of system has a number of disadvantages, notably that the isolated coaxial cables do not provide a suitable life-like radio path that is susceptible to any number of problematic elements as in an actual operating environment. A more detailed discussion of a coaxial network for simulating the air interface is described in U.S. Pat. No. 5,465,393 granted to Frostrom et al. entitled: "Simulated Air Interface System for Simulating Radio Communication", issued on Nov. 7, 1995 to the present assignee and is incorporated by reference herein in its entirety.
FIG. 1 illustrates a typical prior art coaxial network testing system for use in simulating the air interface for test operations. A plurality of mobiles, referred collectively by the reference numeral 100, are each connected to a base station 110 via individual coaxial cables 120. The connection is made by connecting one end of coaxial cable 120 to the antenna port of each of the mobiles and connecting the other end to base station 110. The cables provide individual shielded radio paths to and from each of the mobiles to the base station thus allowing a large number of mobiles to be conveniently tested from one location. In addition, very precise control of the signal strengths to and from the mobile can be achieved thereby providing suitable conditions for repeat testing. A computer 130 is connected via link 140 to each of the mobiles in order to automate the testing procedure and monitor and record the results.
A disadvantage of using a coaxial network is that the same radio path is used for both the transmission and reception of signals thereby unrealistically isolating the mobile from signals from other mobiles i.e. co-channel interference. This is not suitable for testing problems arising from the interaction of signals from other mobiles such as mass traffic testing. Mass traffic testing includes testing the system during mass registrations and mass call setups by a large number of mobiles which better simulates real-life situations. This has become an area of growing importance since sustained high traffic conditions may lead to access collisions arising from a large number of mobiles attempting to simultaneously access a limited bandwidth channel. Thus a coaxial based network is unsuitable for this type of testing.
In view of the foregoing, it is desirable to provide a testing platform that more closely approximates real-life conditions with regards to the air interface. A further desire is to provide a test environment that maintains consistent conditions which permit repeat testing.